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The temporal resolution

The HelioClim-1 (HC1) database has been estimated by applying the Heliosat-2 method on the a reduced dataset from the Meteosat First Generation satellite (Lefèvre et al. 2007). Indeed, in the past, Meteosat images were for pay and very expensive. As well, all the images at full resolution represented a large volume of data to store, so MINES ParisTech decided to work on a reduced set. Only one every 6 images was conserved, increasing the temporal resolution from 30 min up to 3 hours, from which only the daily means have been conserved and provided to the users.

The spatial resolution: the B2 image dataset

The pixel of the Meteosat First Generation satellite image in the visible range is 2.5 km at nadir (against 3 km for Meteosat Second Generation, the map of MSG is identical for MFG if it is multiplied by 2.5/3). The thermal channel (Water Vapor) of MFG has a pixel size of 5 km at nadir. There is a perfect superposition between this "big pixel" of this channel and the 4 pixels in the visible range. In the literature, one can regularly find in the literature a reference to 5 km images in the visible ranges, which correspond to an average of these 4 pixels which are coincident with the thermal range ones.

From these 5 km images in the visible ranges, degraded version images have been derived, named B2; this set has been built taking one pixel every 6 in lines and columns. These images are referred as (20 or) 30 km images. HelioClim-1 has been built from these images B2.

As a consequence, the spatial resolution was reduced from 2.5 km down to approx. 20-30 km.

What is stored in the database?

The database contained the daily GHI values. Decomposition models and post-processing layers provide additional data. In particular, to derive the monthly HelioClim-1 values over a normal plane (HC1monthDNI) from the original daily HelioClim-1 values (HC1day) values, the algorithm of Collares-Peirera and Rabl (1979) is used to obtain intra-day radiation values over the horizontal plane. Then another model is applied to obtain the components of the radiation over the normal surface, before aggregation to obtain the monthly values.

Missing values in HC1: Several slots (or images) were regularly missed, due to internal shortage or interruption of service by Eumetsat. Except is the whole images of a day are missing, this is totally invisible for the users since the images of a day are interpolated if at least one image is available. For you information, the missing slots are available on this page with the corresponding information, if available (missing value in HelioClim: -999). Please note that the field of view is restricted to the pixels belonging to the Earth and for which the elevation angle of the sun above horizon is larger than 12 degrees.

Hypothesis "end of time integration": the date or instant given for each value corresponds to the end of the summarization/time integration. I.e. 24/04/2010 08:00 means that the associated irradiation or irradiance value has been measured between 07:00 and 08:00. This convention addresses the standard recommanded by the WMO (World Meteorological Organisation).

HelioClim-1 and Digital Elevation Model

Topography impacts the solar radiation values at ground levels in two ways: the variation of the optical path length and the shadowing effect due to the horizon. Indeed, according to the atmospheric layer tickness crossed by sun rays when reaching the ground, the radiation is modulated: the higher the elevation of a point, the narrower the atmospheric optical layer, and finally the higher the radiation. Each HelioClim-1 value is stored using the default elevation database TerrainBase 5' (worlwide database, spatial resolution of 5' of arc, equivalent to approx. 10 km). No correction in height is applied to take into account the exact height of the point requested by the user. It is also not possible to take into account the shadowing effect due to the far horizon.